1. Field of the Invention
This invention relates to a display device, especially to a display device with a DA conversion function converting a digital image signal into an analog image signal.
2. Description of the Related Art
An electroluminescent (referred to as EL hereinafter) display device with an EL element has been gathering attention as a display device substituting a CRT or an LCD. The development effort for the EL display device with a thin film transistor (referred to as TFT hereinafter) as a switching element for driving the EL element has been made accordingly.
FIG. 7 is an equivalent circuit diagram of a pixel of an organic EL display panel. A gate signal line 50 supplying a gate signal Gn and a drain signal line 60 supplying a drain signal, a video signal Dm, cross each other. The video signal Dm is produced by sampling a video signal by using a sampling signal.
An organic EL element 120, a TFT 100 for driving the organic El element 120, and a TFT 110 for selecting the pixel are disposed near the crossing of the signal lines.
A positive source voltage PVdd is applied to the drain 100d of the driving TFT 100 of the organic EL element. And the source 100s is connected to an anode 121 of the organic EL element 120.
The gate 110g of the TFT 110 for selecting pixel is provided with the gate signal Gn by being connected to the gate signal line 50, and provided with the video signal Dm by being connected to the drain signal line 60. The source 110s of the TFT 110 is connected to the gate 100g of the TFT 100. The gate signal Gn is generated from a gate driver circuit (not shown in the figure). The video signal Dm is outputted from a drain driver circuit (not shown in the figure).
The organic EL element 120 includes the anode 121, a cathode 122 and an emissive layer 123 inserted between the anode 121 and the cathode 122. The cathode 122 is provided with a negative source voltage CV.
A storage capacitance element 130 is connected to the gate 100g of the TFT 100. That is, one of the electrodes of the storage capacitance element 130 is connected to the gate 100g, and the other electrode is connected to a storage capacitance electrode 131. The storage capacitance element 130 is disposed in order to keep the video signal of the pixel for one field period by keeping the charge corresponding to the video signal Dm.
The operation of the display device with the above configuration is as follows. The TFT 110 turns on when the gate signal Gn becomes a high level for one horizontal period. Then the video signal Dm is supplied from the drain signal line 60 to the gate 100g of the TFT 100 through the TFT 110. The conductance of the TFT 100 changes according to the video signal supplied to the gate 100g and the corresponding driving electric current is supplied to the organic EL element 120 through the TFT 100, which results in an illumination of the organic EL element 120.
An analog image signal inputted to the drain signal line 60 is obtained by converting the inputted digital image signal into the analog image signal by a D/A converter. Conventional display devices with a D/A converter built inside the display panel usually have the D/A converter near the driver circuit disposed in the peripheral area of the pixels.
However, since the D/A converter is disposed near the driver circuit, the conventional display device has complicated circuit designs in the peripheral area of the pixels, leading to an enlarged framing area of the display panel to accommodate the D/A converter.